Electrosurgical instrument with axially-spaced electrodes

ABSTRACT

A bi-polar electrocautery needle comprising an inner electrode on outer electrode and recoverable insulating-locking member for insulting the electrode from one another and locking them into relative position to one another. And the method of making the bi-polar electrocautery needle in accordance with this invention, the steps including: expanding recoverable dielectric material over an inner electrode; and recovering the material between the electrodes for insulating and locking the electrodes into relative position with one another.

This application is a division of and claims the benefit of U.S.application No. 09/258,516/Feb. 26, 1999 now abandoned which is adivision of 08/761,096/Dec. 5, 1996 now U.S. Pat. No. 6,312 408, whichis division of 08/446,767/Jun. 2, 1995 now U.S. Pat. No. 5,697,909,which is a continuation-in-part 08/059,681/May 10, 1993, now abandoned,the disclosure of which is incorporated by referenced.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of electrosurgeryand, more particularly, to surgical devices and methods which employvery high frequency electrodes comprising an array of individual,isolated electrode terminals.

The field of electrosurgery includes a number of loosely relatedsurgical techniques which have in common the application of electricalenergy to modify the structure or integrity of patient tissue.Electrosurgical procedures usually operate through the application ofvery high frequency currents to cut or ablate tissue structures, wherethe operation can be monopolar or bipolar. Monopolar techniques rely onexternal grounding of the patient, where the surgical device definesonly a single electrode pole. Bipolar devices comprise both electrodesfor the application of current between their surfaces.

Electrosurgical procedures and techniques are particularly advantageoussince they generally reduce patient bleeding and trauma associated withcutting operations. Additionally, electrosurgical ablation procedures,where tissue surfaces and volume may be reshaped, cannot be duplicatedthrough other treatment modalities.

The use of electrosurgical procedures in electrically conductiveenvironments, however, can be problematic. For example, manyarthroscopic procedures require flushing of the region to be treatedwith isotonic saline (also referred to as normal saline), both tomaintain an isotonic environment and to keep the field of viewing clear.The presence of saline, which is a highly conductive electrolyte, cancause shorting of the electrosurgical electrode in both monopolar andbipolar modes. Such shorting causes unnecessary heating in the treatmentenvironment and can further cause non-specific tissue destruction.

Present electrosurgical techniques used for tissue ablation also sufferfrom an inability to control the depth of necrosis in the tissue beingtreated. Most electrosurgical devices rely on creation of an electricarc between the treating electrode and the tissue being cut or ablatedto cause the desired localized heating. Such arcs, however, often createvery high temperatures causing a depth of necrosis greater than 500 μm,frequently greater than 800 μm, and sometimes as great as 1700 μm. Theinability to control such depth of necrosis is a significantdisadvantage in using electrosurgical techniques for tissue ablation,particularly in arthroscopic procedures for ablating and/or reshapingfibrocartilage, articular cartilage, meniscal tissue, and the like.

In an effort to overcome at least some of these limitations ofelectrosurgery, laser apparatus have been developed for use inarthroscopic and other procedures. Lasers do not suffer from electricalshorting in conductive environments, and certain types of lasers allowfor very controlled cutting with limited depth of necrosis. Despitethese advantages, laser devices suffer from their own set ofdeficiencies. In the first place, laser equipment can be very expensivebecause of the costs associated with the laser light sources. Moreover,those lasers which permit acceptable depths of necrosis (such as excimerlasers, erbium:YAG lasers, and the like) provide a very low volumetricablation rate, which is a particular disadvantage in cutting andablation of fibrocartilage, articular cartilage, and meniscal tissue.The holmium:YAG and Nd:YAG lasers provide much higher volumetricablation rates, but are much less able to control depth of necrosis thanare the slower laser devices. The CO₂ lasers provide high rate ofablation and low depth of tissue necrosis, but cannot operate in aliquid-filled cavity.

For these reasons, it would be desirable to provide improved apparatusand methods for efficiently cutting and ablating tissue, particularlyfibrocartilage, articular cartilage, meniscal tissue, and the like inarthroscopic and other procedures. Such apparatus and methods should beable to selectively cut and ablate tissue and other body structures inelectrically conductive environments, particularly regions which arefilled with blood, irrigated with saline, or the like. Such apparatusand methods should be able to perform cutting and ablation of tissues,particularly fibrocartilage, articular cartilage, meniscal tissue, andthe like, while limiting the depth so necrosis and tissue adjacent tothe treatment site. Such apparatus and methods should be amenable toprecise control over the energy flux levels applied to the treatmentregion, and should be able to provide energy densities sufficient toprovide rapid cutting and ablation. The devices should be adaptable to awide variety of purposes, particularly including both small and largeelectrode surfaces, and rigid and flexible structures which can be usedin open surgery, arthroscopic surgery, and other minimally invasivesurgical techniques.

2. Description of the Background Art

Devices incorporating radio frequency electrodes for use inelectrosurgical and electrocautery techniques are described in Rand etal. (1985) J. Arthro. Surg. 1:242-246 and U.S. Pat. Nos. 5,281,216;4,943,290; 4,936,301; 4,593,691; 4,228,800; and 4,202,337. U.S. Pat. No.5,281,216 describes a bipolar device having an active electrode coatedwith a high impedance material where the differential impedance betweenthe active and return electrodes is optimized to provide a desiredcutting effect. Vascular catheters and devices incorporating radiofrequency electrodes to assist in penetrating atheroma and plaque aredescribed in U.S. Pat. Nos. 5,281,218; 5,125,928; 5,078,717;4,998,933;and 4,976,711, and PCT publications WO 93/20747 and WO 90/07303, thelatter of which describes a catheter having four isolated electrodesurfaces at its distal end. Electrosurgical power supplies includingpower controls and/or current limiting systems are described in U.S.Pat. No. 5,267,997 and PCT publication WO 93/20747. Surgical lasers forcutting and ablation in arthroscopic and other procedures are describedin Euchelt et al. (1991) Surgery and Medicine II: 271-279; and U.S. Pat.Nos. 5,147,354; 5,151,098; 5,037,421; 4,968,314; 4,785,806; 4,737,678;4,736,743; and 4,240,441.

SUMMARY OF THE INVENTION

The present invention provides methods and apparatus for selectivelyapplying electrical energy to structures within a patient's body. Themethods and apparatus are particularly useful for performingelectrosurgical interventions, such as ablation and cutting of bodystructures, through the controlled application of high frequencyelectrical voltages and currents.

Apparatus according to the present invention comprise electrosurgicalprobes including a shaft having a proximal end, a distal end, anelectrode array disposed near the distal end of the shaft, and aconnector disposed near the proximal end of the shaft. The shaft will beof a type suitable for use in open and/or minimally invasive surgicalprocedures, such as arthroscopic, laparoscopic, thoracoscopic, and otherendoscopic procedures. The shaft may be rigid, flexible, or include bothrigid and flexible portions, and will be generally suitable formanipulation by the treating physician from the proximal end. A commonelectrode may optionally be provided on the shaft, typically beingmounted over the exterior of the shaft and spaced proximally from theelectrode array, and preferably being covered with a perforate,electrically non-conductive shield to protect against accidental tissuecontact. The electrode array includes a plurality of electricallyisolated electrode terminals disposed over a contact surface, which maybe a planar or non-planar surface and which may be located at the distaltip or over a lateral surface of the shaft, or over both the tip andlateral surface(s). Such electrode arrays are particularly useful forperforming electrosurgical ablation, as described in more detail below.In addition to planar and other surfaces, the electrode array may bearranged in a linear pattern, which is particularly useful as a bladefor electrosurgical cutting procedures. The electrode array will includeat least two and preferably more electrode terminals, and may furthercomprise a temperature sensor. The connector permits electrical couplingof the electrode terminals, and optionally temperature sensor, to a highfrequency power supply and optionally temperature monitor and/orcontroller or operation of the probe.

The use of such electrode arrays in electrosurgical procedures isparticularly advantageous as it has been found to limit the depth oftissue necrosis without substantially reducing power delivery andablation rates. Heretofore, increased power delivery withelectrosurgical devices has generally been achieved by increasingmonolithic electrode area. The resulting large electrode surfaces,however, cause tissue necrosis to a depth which varies proportionallywith the width and area of the electrode surface. The present inventionprovides a more controlled necrosis depth by utilizing a plurality ofisolated electrode terminals, where the terminals are preferablylaterally spaced-apart by a distance from one-tenth to one terminaldiameter, with spacing between larger electrode terminals generallybeing at the lower end of the range. Such spacing provides adequatepower delivery and ablation rates without excessive tissue necrosis,which is usually limited to a depth less than one electrode terminaldiameter.

Apparatus according to the present invention further include anelectrosurgical high frequency power supply comprising a multiplicity ofindependent current sources and a connector which mates with acorresponding connector on the electrosurgical probe. The currentsources preferably comprise passive or active current limiting circuitstructures in parallel with each other and in series with a commonvoltage source within the power supply. Passive current limiting circuitstructures may include inductor(s), capacitor(s), and/or resistor(s) inknown circuit configurations. In all cases, the passive current limitingstructures will be designed to limit current flow when the associatedelectrode terminal is in contact with a low resistance return path backto the common or return electrode. Preferred passive current limitingstructures comprise (1) inductors in series with each electrode terminaland (2) capacitors in series and inductors in parallel with eachelectrode terminal, as described in detail hereinafter.

Active current limiting circuit structures will usually comprise aswitching element to turn off current flow whenever the associatedelectrode terminal contacts a low (or in some instances high) impedancereturn path back to the common or return electrode. The switchingelement could be mechanical, e.g., a relay, but preferably will be solidstate, e.g., a silicon controlled rectifier (SCR) or silicon controlledswitch (SCS). The switch will be turned on and off by a controller whichcan detect the low resistance path (typically by sensing current flowabove a threshold value). The controller can be implemented in hardwareor software, typically being part of the power supply.

The high frequency electrosurgical power supply optionally includes atemperature controller which is connected to the temperature sensor onthe electrosurgical probe and which adjusts the output voltage of thevoltage source in response to a temperature set point and the measuredtemperature value received from the probe. In this way, the power outputand temperature may be controlled while the individual current sourceslimit or block the power output from corresponding individual electrodeterminals. Such limitation of individual electrode terminal poweroutputs is critical to limiting energy loss from the electrode array asdescribed in more detail below.

The present invention still further provides an electrosurgical systemincluding both the electrosurgical probe and electrosurgical powersupply as described above.

According to the method of the present invention, an electrosurgicalprobe is positioned adjacent to a body structure so that an electrodearray is brought into at least partial contact with the structure. Theelectrode array includes a plurality of isolated electrodes, and a highfrequency voltage is applied between the electrode array and thepatient's body. The voltage causes current flow between each electrodeterminal and the body structure which is through all low electricalimpedance paths is preferably but not necessarily limited. It will beappreciated that such low impedance paths generally occur when anelectrode terminal does not contact the body structure, but rather is incontact with a low impedance environment, such as saline, blood, orother electrolyte. The presence of an electrolyte provides a relativelylow impedance path back to the common or return electrode, which may beon the electrosurgical probe or may be attached externally to thepatient. Such electrosurgical methods are particularly useful when aregion is to be flushed with saline, such as in an electrosurgicalablation of fibrocartilage, articular cartilage, meniscal tissue, andthe like, in arthroscopic procedures.

In some cases, it may be desirable to provide current limitation orcontrol when individual electrode terminals contact very high resistancebody structures, such as bone, cartilage (which has a higher resistivitythan meniscus and other tissues), and the like. Current limitation whenthe electrode terminals contact high resistance structures will usuallyrequire active control schemes (i.e., passive control circuitry will beinadequate), and it will be possible to provide control protocols wherecurrent can be limited when it either exceeds or falls below an expectedrange characteristic of the target tissue to be treated.

A further understanding of the nature and advantages of the inventionwill become apparent by reference to the remaining portions of thespecification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the electrosurgical system including anelectrosurgical probe and electrosurgical power supply constructed inaccordance with the principle of the present invention.

FIG. 2 is an enlarged, detailed view of the distal tip of theelectrosurgical probe of FIG. 1.

FIG. 3 is a cross-sectional view of the distal tip of theelectrosurgical probe of FIGS. 1 and 2.

FIG. 4 is a schematic view of a particular connector and leadarrangement which can be employed in the electrosurgical probe of FIGS.1-3.

FIG. 5 is a detailed cross-sectional view of the distal end of anelectrosurgical probe illustrating an electrode arrangement suitable forrapid cutting and ablation or tissue structures.

FIG. 6 is a detailed left-end view of the distal end of theelectrosurgical probe of FIG. 5.

FIG. 7 is a detailed cross-sectional view of the distal end of anelectrosurgical probe illustrating an electrode arrangement suitable forsmoothing of tissue structures.

FIG. 8 is a perspective view of an electrosurgical probe with theelectrode array disposed at a right angle to the axis of shaft of probe.

FIG. 9 is a perspective view of an electrosurgical probe with electrodearrays disposed on the lateral and tip surfaces at the distal end of theprobe.

FIG. 10 is a perspective view of the distal end of an electrosurgicalprobe with an atraumatic shield extending distally from the electrodearray.

FIG. 11 illustrates use of the probe of FIG. 10 in ablating targettissue.

FIG. 12 is a detailed end view of an electrosurgical probe having anelongate, linear array of electrode terminals suitable for use insurgical cutting.

FIG. 13 is a detailed view of a single electrode terminal having aflattened end at its distal tip.

FIG. 14 is a detailed view of a single electrode terminal having apointed end at its distal tip.

FIG. 15 is a detailed view of a single electrode terminal having asquare end at its distal tip.

FIGS. 16 and 16A are electrical schematic diagrams illustrating twoembodiments or the circuitry of a high frequency power supplyconstructed in accordance with the principles of the present invention.

FIG. 17 is a perspective view of the distal end of an electrosurgicalprobe having an elongate, linear array of electrode terminals.

FIG. 18 is a perspective view of the distal end of an electrosurgicalprobe having a deflectable distal tip.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a method and apparatus for selectivelyheating a target location within a patient's body, such as solid tissueor the like, particularly including articular cartilage, fibrocartilage,meniscal tissue, and the like. In addition to articular cartilage andfibrocartilage, tissues which may be treated by the method and apparatusof the present invention include tumors, abnormal tissues, and the like.For convenience, the remaining disclosure will be directed specificallyat the cutting, shaping or ablation of fibrocartilage and articularcartilage during arthroscopic or endoscopic procedures but it will beappreciated that the apparatus and methods can be applied equally wellto procedures involving other tissues of the body, as well as to otherprocedures including open surgery, laparoscopic surgery, thoracoscopicsurgery, and other endoscopic surgical procedures.

The target tissue will be, by way of example but not limited to,articular cartilage, fibrocartilage, and meniscal tissue, such as foundin the joints of the knee, shoulder, hip, foot, hand and spine. Thepresent invention uses an electrode array including a plurality ofindependently current-limited and/or power-controlled electrodeterminals distributed over a distal contact surface of a probe to applyheat selectively to the target tissue while limiting the unwantedheating of the surrounding tissue and environment resulting from powerdissipation into surrounding electrically conductive liquids, such asblood, normal saline, and the like.

The electrosurgical probe will comprise a shaft having a proximal endand a distal end which supports an electrode array near its distal end.The shaft may assume a wide variety of configurations, with the primarypurpose being to mechanically support the electrode array and permit thetreating physician to manipulate the array from a proximal end of theshaft. Usually, the shaft will be a narrow diameter rod or tube, moreusually having dimensions which permit it to be introduced through anassociated trocar or cannula in a minimally invasive procedure, such asarthroscopic, laparoscopic, thoracoscopic, and other endoscopicprocedures. Thus, the shaft will typically have a length of at least 10cm, more typically being 25 cm, or longer, and will have a diameter ofat least 1 mm, usually being at least 2 mm, and frequently being in therange from 2 to 10 mm. The shaft may be rigid or flexible, with flexibleshafts optionally being combined with a generally rigid external tubefor mechanical support. Flexible shafts may be combined with pull wires,shape memory actuators, and other known mechanisms for effectingselective deflection of the distal end of the shaft to facilitatepositioning of the electrode array. The shaft will usually include aplurality of wires or other conductive elements running axiallytherethrough to permit connection of the electrode array to a connectorat the proximal end of the shaft. Specific shaft designs will bedescribed in detail in connection with the figures hereinafter.

The electrode array have an area in the range from 0.01 mm² to 2.5 cm²,preferably from 0.025 mm² to 1 cm², more preferably from 0.25 mm² to 50mm², and often from 0.5 mm² to 25 mm², and will usually include at leasttwo isolated electrode terminals, more usually at least four electrodeterminals, preferably at least six electrode terminals, more preferablyat least eight electrode terminals, even more preferably at least 15electrode terminals, and still more preferably at least 20 electrodeterminals, and often 50 or more electrode terminals, disposed over thedistal contact surfaces on the shaft. By contacting the electrodearray(s) on the contact surface(s) against target tissue and applyinghigh frequency voltage between the array(s) and an additional common orreturn electrode in direct or indirect contact with the patient's body,the target tissue is selectively ablated or cut, permitting selectiveremoval of portions of the target tissue while desirably minimizing thedepth of necrosis to surrounding tissue. In particular, this inventionprovides a method and apparatus for effectively ablating and cuttingarticular cartilage and fibrocartilage by simultaneously applying both(1) electrical energy to the target tissue surrounding and immediatelyadjacent to the tip of the probe and (2) pressure against the targettissue using the probe itself.

Each individual electrode terminal in the electrode array iselectrically insulated from all other electrode terminals in the arraywithin said probe and is connected to a power source which is isolatedfrom each of the other electrodes in the array or to circuitry whichlimits or interrupts current flow to the electrode when low resistivitymaterial (e.g., blood or electrically conductive saline irrigant) causesa lower impedance path between the common electrode and the individualelectrode terminal. The isolated power sources for each individualelectrode may be separate power supply circuits having internalimpedance characteristics which limit power to the associated electrodeterminal when a low impedance return path is encountered or may be asingle power source which is connected to each of the electrodes throughindependently actuable switches.

The tip region of the probe is thus composed of many independentelectrode terminals designed to deliver electrical energy in thevicinity of the tip. The selective heating of the target tissue isachieved by connecting each individual electrode terminal and the commonelectrode (e.g., a band of conductive material proximal to the electrodearray at the tip or an external electrode which is placed on the outsideof the patient's body) to a power source having independently controlledor current-limited channels The application of high frequency voltagebetween the common electrode and the electrode array results in theconduction of high frequency current from each individual electrodeterminal to the said common electrode. The current flow from eachindividual electrode terminal to the common electrode is controlled byeither active or passive means, or a combination thereof, to deliverelectrical energy to the target tissue while minimizing energy deliveryto surrounding (non-target) tissue and any conductive fluids which maybe present (e.g., blood, electrolytic irrigants such as saline, and thelike).

In a preferred aspect, this invention takes advantage of the differencesin electrical resistivity between the target tissue (e.g., articularcartilage or fibrocartilage) and the surrounding conductive liquid(e.g., isotonic (normal) saline irrigant). By way of example, for anyselected level of applied voltage, if the electrical conduction pathbetween the common electrode and one of the individual electrodeterminals within the electrode array is isotonic saline irrigant liquid(having a relatively low electrical resistivity), said current controlmeans connected to the individual electrode will limit current flow sothat the heating of intervening conductive liquid is minimized. Incontrast, if a portion of or all of the electrical conduction pathbetween the common electrode and one of the individual electrodeterminals within the electrode array is articular cartilage orfibrocartilage (having a relatively higher electrical resistivity), saidcurrent control circuitry or switch connected to said individualelectrode will allow current flow sufficient for the heating or ablationor electrical breakdown of the target tissue in the immediate vicinityof the electrode surface.

The application of a high frequency voltage between the common or returnelectrode and the electrode array for appropriate time intervals effectsablation, cutting or reshaping of the target tissue. The tissue volumeover which energy is dissipated (i.e., a high voltage gradient exists)may be precisely controlled, for example, by the use of a multiplicityof small electrodes whose effective widths (i.e., diameters for roundwire terminals) range from about 0.05 mm to 2 mm, preferably from about0.1 mm to 1 mm. Electrode terminal areas for both circular andnon-circular terminals will have a contact area below 5 mm², preferablybeing in the range from 0.001 mm² to 2 mm², and more preferably from0.01 mm² to 1 mm². The use of small electrode terminals reduces theextent and depth of tissue necrosis as a consequence of the divergenceof current flux lines which emanate from the exposed surface of eachelectrode terminal. Energy deposition in tissue sufficient forirreversible damage (i.e., necrosis) has been found to be limited to adistance of about one-half to one electrode terminal diameter. This is aparticular advantage over prior electrosurgical probes employing singleand/or larger electrodes where the depth of tissue necrosis may not besufficiently limited. Heretofore, increased power application andablation rates would usually be achieved by increasing the electrodearea. Surprisingly, with the present invention, it has been found thatthe total electrode area can be increased (to increase power deliveryand ablation rate) without increasing depth of necrosis by providingmultiple small electrode terminals. Preferably, the terminals will bespaced-apart by a distance in the range from one-tenth diameter to onediameter for optimum power delivery, with smaller spacing between largerterminals. The depth of necrosis may be further controlled by switchingthe applied voltage off and on to produce pulses of current, said pulsesbeing of sufficient duration and associated energy density to effectablation and/or cutting while being turned off for periods sufficientlylong to allow for thermal relaxation between energy pulses. In thismanner, the energy pulse duration, magnitude and the time intervalbetween energy pulses are selected to achieve efficient rates of tissueablation or cutting while allowing the temperature of the heated zone oftissue to “relax” or return to normal physiological temperatures beforethe onset of the next energy (current) pulse.

The rate of energy delivery to the target tissue is controlled by theapplied voltage level and duty cycle of the voltage pulse. The use ofhigh frequency current minimizes induced stimulation of muscle tissue ornerve tissue in the vicinity of the body structure being treated. Inaddition, high frequency minimize the risk of interfering with thenatural pacing of the heart circumstances where the probe of the presentinvention is used near the heart.

The power applied to the common electrode and the electrode array willbe at high or radio frequency, typically between about 20 kHz and 20MHz, usually being between about 30 kHz and 1 MHz, and preferably beingbetween about 50 kHz and 400 kHz. The RMS (root mean square) voltageapplied will usually be in the range from about 5 volts to 1000 volts,preferably being in the range from about 50 volts to 800 volts, and morepreferably being in the range from about 10 volts to 500 volts. Usually,the current level will be selectively limited or controlled and thevoltage applied will be independently adjustable, frequently in responseto the resistance of tissues and/or fluids in the pathway between anindividual electrode and the common electrode. Also, the applied currentlevel may be in response to a temperature control means which maintainsthe target tissue temperature with desired limits at the interfacebetween the electrode arrays and the target tissue. The desired surfacetemperature of the target tissue will usually be in the range from about40° C. to 500° C., and more usually from about 50° C. to 300° C.

The preferred power source of the present invention delivers a highfrequency current selectable to generate average power levels rangingfrom tens of milliwatts to tens of watts per electrode, depending on thetarget tissue being heated, the rate of ablation desired or the maximumallowed temperature selected for the probe tip. The power source allowsthe user to select the current level according to the specificrequirements of a particular arthroscopy procedure or other endoscopicprocedure.

The power source will be current limited or otherwise controlled so thatundesired heating of electrically conductive fluids or other lowelectrical resistance tissues does not occur. In a presently preferredembodiment of the present invention, current limiting inductors areplaced in series with each independent electrode terminal, where theinductance of the inductor is selected to provide high impedance at thefrequency of operation. Alternatively, capacitor-inductor (LC) circuitstructures may be employed, as described in more detail below.Additionally, a current limiting resistor may be selected having a largepositive temperature coefficient of resistance so that, as the currentlevel begins to rise for any individual electrode in contact with a lowresistance medium (e.g., saline irrigant), the resistance of the currentlimiting resistor increases significantly, thereby minimizing the powerdelivery from said electrode into the low resistance medium (e.g.,saline irrigant). Thus, the electrode terminal sees a relativelyconstant current source so that power dissipation through a lowresistance path, e.g., normal saline irrigant, will be substantiallydiminished.

As an alternative to such passive circuit structures, constant currentflow to each electrode terminal may be provided by a multi-channel powersupply. A substantially constant current level for each individualelectrode terminal within a range which will limit power deliverythrough a low resistance path, e.g., isotonic saline irrigant, would beselected by the user to achieve the desired rate of cutting or ablation.Such a multi-channel power supply thus provides a constant currentsource with selectable current level in series with each electrodeterminal, wherein all electrodes will operate at or below the same, userselectable maximum current level. Current flow to all electrodeterminals could be periodically sensed and stopped if the temperaturemeasured at the surface of the electrode array exceeds user selectedlimits. Particular control system designs for implementing this strategyare well within the skill of the art.

Yet another alternative involves the use of one or several powersupplies which allow one or several electrodes to be simultaneouslyenergized and which include active control means for limiting currentlevels below a preselected maximum level. In this arrangement, only oneor several electrodes would be simultaneously energized for a briefperiod. Switching means would allow the next one or several electrodesto be energize for a brief period. By sequentially energizing one orseveral electrodes, the interaction between adjacent electrodes can beminimized (for the case of energizing several electrode positioned atthe maximum possible spacing within the overall envelope of theelectrode array) or eliminated (for the case of energizing only a singleelectrode at any one time). As before, a resistance measurement meansmay be employed for each electrode prior to the application of powerwherein a (measured) low resistance (below some preselected level) willprevent that electrode from being energized during given cycle. By wayof example, the sequential powering and control scheme of the presentinvention would function in a manner similar to an automobiledistributor. In this example, an electrical contact rotates pastterminals connected to each spark plug. In this example, each spark plugcorresponds to the exposed surface of each of the electrodes. Inaddition, the present invention includes the means to measure theresistance of the medium in contact with each electrode and causevoltage to be applied only if the resistance exceeds a preselectedlevel.

The electrode array is formed over a contact surface on the shaft of theelectrosurgical probe. The area of the contact surface can vary widely,and the contact surface can assume a variety of geometries, withparticular areas in geometries being selected for specific applications.Electrode array contact surfaces will have areas as set forth above andcan be planar, concave, convex, hemispherical, conical, or virtually anyother regular or irregular shape. Most commonly, the electrode arrayswill be formed at the distal tip of the electrosurgical probe shaft,frequently being planar, disk-shaped, or hemispherical surfaces for usein reshaping procedures or being linear arrays for use in cutting.Alternatively or additionally, the electrode arrays may be formed onlateral surfaces of the electrosurgical probe shaft (e.g., in the mannerof a spatula), facilitating access to certain body structures inelectrosurgical procedures.

In an exemplary embodiment as shown in FIG. 1, a probe 10 includes anelongated shaft 13 which may be flexible or rigid, with flexible shaftsoptionally being disposed with support cannulas or other structures (notshown). Referring to FIGS. 1 and 2, the probe 10 includes an array ofelectrode terminals 58 disposed on the distal tip 12 of shaft 13. Theelectrode terminals 58 are electrically isolated from each other andfrom a common or return electrode 17 which is disposed on the shaftproximally of the electrode array, preferably being with 1 mm to 25 mmof the distal tip 12. Proximally from the tip 12, the return electrode17 is generally concentric with the shaft of the probe 10. Said probe 10may be constructed having zones of varying flexibility (or conversely,stiffness) along the probe length. It is advantageous, for example, tohave greater flexibility (i.e., lesser stiffness) at the distal end ofthe probe 10 (region L₃ in. FIG. 1) in order to increase the capabilityof the probe to gain access to operative sites which are not in a directline path with the entrance site into the body cavity. In the preferredembodiment illustrated in FIG. 1, probe 10 would have two or threeregions wherein regions L₁ and L₂ are stiffer than region L₃. Thepreferred length for region L₁ is in the range from 0.5 mm to 25 mm, forregion L₂ is in the range from 1 mm to 20 mm, and for region L₃ is from5 cm to 25 cm.

Still referring to FIGS. 1 and 2, each of the terminals 58 is connectedto an active or passive control network within a power source andcontroller 28 by means of the individually insulated conductors 42. Theproximal portion of the probe 10 is also equipped with a connector 19which can be removably connected to a connector 20 in a reusable handle22. The proximal portion of the handle 22 and cable 24 also has aconnector 26 for providing the electrical connections to the controller28.

Referring to FIG. 1, the power source and controller 28 provides highfrequency voltage to the electrode terminals 28 (of FIG. 2) by means ofa cable 24 from connector 20 in handle 22 to receptacle 26, the powersource and controller 28. The power source and controller 28 has aselector 30 to change the applied voltage level. A conductor 44 extendsfrom the common electrode 17 (FIG. 2) and is connected to the powersource and controller 28 by the same cable 24. The power source andcontroller 28 also includes the means for energizing the electrodes 58of probe 10 through the depression of foot pedal 39 in a foot pedal 37Positioned close to the user. The assembly 37 may also include a secondpedal not shown) for remotely adjusting the energy level applied toelectrodes 58.

Referring to FIG. 2 and FIG. 3, the distal tip 12 of probe 10 in thepreferred embodiment contains the exposed surfaces of the electrodeterminals 58. The electrode terminals 58 are secured in a matrix 48 ofsuitable insulating material (e.g., ceramic or glass) which could beformed at time of manufacture in a flat, hemispherical or other shapeaccording to the requirements of a particular procedure. Proximal to thedistal tip 12, the isolated electrode wires 42 are contained in aninsulating insert 14 (FIG. 3) of generally cylindrical shape extendingfrom matrix 48 and into a tubular support member 56.

Referring to FIGS. 2 and 3, the tubular support member 56 is preferablyformed from an electrically conductive material, usually metal, and isdisposed within an electrically insulative jacket 18. The electricallyconductive tubular support member 56 defines the common or returnelectrode 17 with respect to the array of individual electrodes 12 inorder to complete the electrical circuit such that current will flowbetween each individual electrode 58 and the common electrode structure17. The common electrode 17 is located near the distal tip 12 of probe10. The insulating insert 14 distal to the common electrode 17 iscomposed of an electrically insulating material such as epoxy, plastic,ceramic, glass or the like. The electrically conductive tubular supportmember 56 will preferably be sufficiently rigid to provide adequatecolumn strength to manipulate body structures with the shaft of probe10. The tubular member 56 is composed of a material selected from thegroup consisting of stainless, titanium or its alloys, molybdenum or itsalloys, and nickel or its alloys. The electrically conductive tubularmember 56 will preferably be composed of the same metal or alloy whichforms the electrode terminals 58 to minimize any potential for corrosionor the generation off electrochemical potentials due to the presence ofdissimilar metals contained within an electrically conductive fluid 50such as isotonic saline commonly used as an irrigant for the intendedapplications.

Referring now to FIGS. 2 and 3, the common electrode terminal structure17 includes a perforate shield 16 formed from an electrically insulatingmaterial which is porous or which contains openings which allow asurrounding electrically conducting liquid 50 (e.g., isotonic saline) tocontact the electrically conductive layer 54 which is electricallycoupled to the tubular member 56. As shown in FIG. 3, an annular gap 54may be provided between electrically insulating member 16 and commonelectrode member 56. Proximal to the common electrode region 17, thetubular member 56 is covered over its entire circumference by theelectrically insulating jacket 18, which is typically formed as one ormore electrically insulative sheaths or coatings, such aspolytetrafluoroethylene, polyimide, and the like. The annular gap 54preferably has capillary dimensions to maximize fluid contact even whenthe common electrode terminal structure 17 is not fully immersed in theelectrically conductive liquid.

The provision of the electrically insulative jacket 16 over the commonelectrode structure 17 prevents direct electrical contact between thesurface of tubular member 56 and any adjacent body structure. Suchdirect electrical contact between a body structure (e.g., tendon) and anexposed common electrode member 56 could result in unwanted heating andnecrosis of the structure at the point of contact. As shown in FIG. 3,any contact between the common electrode structure 17 (includingperforate shield 16) and a body structure will only result in thepassage of relatively low current density flux lines 60, therebyminimizing the Joulean heating which could occur in any adjacent bodystructure. Referring to FIGS. 1 and 3, electrical communication betweentubular member 56 and the connector 19 may be provided by anelectrically conducting lead wire 44.

The electrode terminals 58 are electrically insulated from each otherand are secured together in an array by the electrically insulatingmatrix 48. The insulating matrix 48 may be ceramic, glass or otherhigh-temperature insulating material. Proximal to the distal tip 12, theelectrode wires 42 are covered with an electrically insulating material(e.g., polyimide) and are contained in tubular member 56 which extendsthe length of the probe 10. The distal tip 12 end of the probe 10includes the common electrode structure 17 extending over a length L₂which may range from 1 to 20 mm, preferably being from 2 mm to 20 mm. Atip offset L₁ provides a minimum separation between said commonelectrode 17 and the array of electrodes 12, usually being at least 0.5mm, more usually being at least 1 mm, and sometimes being 2 mm orgreater, and preferably being in the range from 0.5 mm to 2 mm.

A central aspect of the present invention is the ability of the probe 10to deliver high energy flux levels selectively only to the intendedareas, i.e., the target tissue T, and not to surrounding healthy tissueor electrically conducting fluids (e.g., isotonic saline irrigant). Suchdirected energy transfer results in selective heating of the targettissue which allows the probe to cut, ablate or recontour the targettissue. Referring to FIGS. 2 and 3, when the electrode array 12 of theprobe 10 is engaged against a region of target tissue 52, some of theelectrode terminals 58 will be in contact with target tissue, whileother electrode terminals may be in contact with electrically conductingfluid 50. Each of the electrode terminals 58 experiences an electricalimpedance which is characteristic of the material which is disposedbetween the individual electrode terminals 58 and the common electrodestructure 17. The present invention takes advantage of the fact that theelectrical impedance (resistivity) of typical target tissue atfrequencies of 50 kHz or greater (e.g., fibrocartilage and articularcartilage) is higher by a factor of approximately four or more than thatof the surrounding electrically conducting fluid 50 typically used as anirrigant during arthroscopic and endoscopic procedures. Thus, if thecurrent passing through each of the electrode terminals 58 is limited toa preselected maximum value, the regions of higher electrical resistancewill generate more Joulean heating (power=I²R, where I is the currentthrough resistance, R) than a region of lower electrical resistance.

In contrast to the present invention, electrosurgical methods andapparatus of the prior art involving a single electrode exhibitsubstantially reduced effectiveness when a portion of the exposedelectrode is in contact with a low-resistance pathway (e.g., isotonicsaline irrigant). In those circumstances, the majority of powerdelivered from the single electrode tip is dissipated within the lowresistance electrically conducting fluid, thereby significantly reducingthe capability to cut or ablate the target tissue.

Furthermore in accordance with the teachings of the present invention,temperature measurement means may be provided in the distal tip 12 tolimit the power delivery if measured temperatures exceed user selectedlevels. Therefore, by either one or a combination of both meansdescribed above the target tissue will be selectively heated up whilethe conductive liquids will experience a minimal rise in temperature.Thus, the probe 10 will selectively and efficiently cut or ablate thetarget tissue.

Still referring to FIG. 3, another aspect of the present invention isthe restriction of high current densities or fluxes to a confined region62 as defined by the current flux lines 60. The confinement of the highcurrent densities to a limited region 62 allows healthy tissue nearby toremain at or near normal physiologic temperatures, thereby limiting thedepth of necrosis into surrounding or underlying healthy tissue 52 to adepth of approximately one electrode diameter.

Alternatively, by energizing only one or several electrode terminals 58at any one time, the depth of necrosis can be still further reducedsince the thermal relaxation time between energy pulses for any specificelectrode will serve to further limit the depth of necrosis.

Referring to FIGS. 1 and 4, the proximal end of probe 10 includes aconnector 19 which includes a multiplicity of electrically conductivepins or contacting member 74 which are in electrical communication witheach electrode wire 42. Said electrical communication may beaccomplished through mechanical crimping of the terminus of connectorpin 74 onto the bare (exposed) electrode lead wire 42 at location 80.Alternatively, the electrode lead wire may be welded, brazed or solderedto the connector pin 74 at location 80. Likewise, the return wire 44electrically communicating with the common electrode member 56 isconnected to one connector pin 76 in a like manner as described abovefor the electrode leads.

The multiplicity of connector pins 74 and 76 are maintained in apredetermined spaced-apart relationship which corresponds to a matingreceptacle 20 at the distal end of the handle 22. The position of thecontact pins 74 and 76 is maintained by an electrically insulativemember 78 which is secured within a connector housing 72 usingadhesives, ultrasonic welding or the like. Alternatively, the connectorhousing may be overmolded around the connector pin assembly 78 andproximal end of the probe shaft member. In the embodiment shown in FIG.4, the electrically conductive tubular member 56 is inserted into thedistal end of the connector 72 and secured using an adhesive or pottingmaterial (e.g., epoxy). The electrically insulative jacket 18 extendsfrom the proximal edge of the common electrode structure 17 to and overan extension 73 at the distal end of the connector housing 72. Theelectrically insulating jacket 18 thereby effects a liquid tight seal atan interface 82 between the jacket 18 and the connector extension 73.The seal prevents the leakage of electrically conductive liquid (e.g.,isotonic saline) into the cavity containing the electrical leads andconnector pins which could result in an electrical short between theelectrodes and/or between any electrode and the common electrode 17.

Still referring to FIG. 4, a sealing means 84 may also be provided atthe proximal end or the connector housing 72 in order to minimize theleakage of electrically conductive liquid (e.g., isotonic saline) at theinterface between the connector 19 and the handle connector 20. Saidseal member 84 may include a conventional elastomeric o-ring placed in asuitably sized o-ring groove within the connector housing 72.

Referring to FIGS. 5 and 6, an embodiment of the present inventiondesigned for rapid ablation of body structures includes a circular array12 of electrode terminals 58 maintained in a spaced-apart relationshipby electrical insulating matrix 48. For convenience, identical numberingfor similar elements will be used for all embodiments. Said electrodeterminals 58 may be fabricated using wires having diameters in thepreferred range set forth above with an electrically insulating coatingextending up to or through the electrically insulating member 48. Theregions of the electrode terminals 58 distal to the distal face of theelectrically insulating matrix 48 are bare (i.e., no electricallyinsulating coating) so that the electrode terminals 58 are directlyexposed to the surrounding electrically conductive liquid (e.g.,isotonic saline) or body structure. The wires and electrode terminals 58will usually be metals or metal alloys, preferably being selected fromthe group comprising titanium, tantalum, molybedeum, tungsten, platinum,rhodium, and alloys thereof. The wires and electrode terminals 58 may besolid or may be composites including a core wire which has been coatedwith one or more of the above metals, compounds or alloys thereof. Theelectrically insulative matrix 48 may be ceramic or glass orglass/ceramic composition (e.g., alumina, borosilicate glass, quartzglass, or the like).

Still referring to FIGS. 5 and 6, the electrode terminals 58 may extenda distance X₃ beyond the distal face of the electrically insulatingmatrix 48, forming protrusions. The extension length X₃ may range from0.05 to 1.0 mm, more preferably being in the range from 0.1 mm to 0.4mm. The interelectrode spacing X₁ ranges from 0.07 mm to 0.4 mm. Theelectrode terminals 58 may be circular, square, rectangular, triangularor polygonal, or irregular in cross-sectional shape. The characteristicdimension D₁ (i.e., diameter in the case of circular electrodes shown inFIG. 6) ranges from 0.1 mm to 0.5 mm depending on the overall size ofthe probe, the rate of ablation required and the maximum allowed depthof necrosis of the body structure being treated. The overall diameter D₂of the electrode array 12 may range from 0.5 mm to 10 mm, morepreferably 1 mm to 5 mm, depending on the particular application andsize of the body structure to be treated. In the case of the circularelectrode array 12 shown in FIG. 6, the electrode terminals 58 arepositioned a small distance X₂ from the perimeter of the electricallyinsulating matrix 48. Said distance X₂ is preferably maintained as smallas practical to maximize the zone of body structure ablated so that itapproximates the diameter D₂ of the distal end of the probe 10, therebypermitting the probe to readily engage the body structure to be ablatedwithout mechanical resistance resulting from an excessive border ordistance X₂ where no ablation has occurred. The distance X₂ ispreferable less than 0.5 mm and more preferably less than 0.3 mm.

Referring to FIG. 7 another embodiment of the present invention intendedfor smoothing of body structures (e.g., articular cartilage located onthe surface of a condyle) while minimizing the depth of necrosis of theunderlying tissue includes electrode terminals 58 in an electricalinsulating matrix 48 is similar to the array shown in FIGS. 5 and 6except that the electrode terminals 58 are flush with the surface of theelectrically insulating matrix 48. The rate of ablation achievable withthe use of “flush” electrode terminals 58 is lower than that forelectrodes which extend beyond the face of the electrically insulativematrix 48, but such flush electrode structure can provide a smoothersurface on the body structure being treated while minimizing the depthof ablation and necrosis.

Referring to FIG. 3, an alternative configuration of the shaft of aprobe 100 is illustrated. This configuration is similar to the electrodearray on distal tip 12 of probe 10 and shaft 14 arrangement shown inFIGS. 5 and 7, except that the shaft 14 near the distal end of probe 100is bent at an angle with respect to the longitudinal axis of the probe.Said angle may range from about 15 degrees to 90 degrees or moredepending on the particular tissue structure to be treated. By way ofexample, the electrode terminal 58 arrangement shown in FIG. 8 allowsthe electrode array to move over a body structure disposed parallel tothe longitudinal axis of the probe 100 with said movement correspondingto forward and backward motion of the probe handle 22. Said electrodeterminals 58 may extend beyond the surface of the electricallyinsulating matrix 48 as shown in FIGS. 5 and 8 or may be flush with theelectrically insulating matrix 48 as shown in FIG. 7.

Yet another embodiment of the electrode array of the present inventionis illustrated in FIG. 9, wherein electrode terminals 58 are disposed ontwo (or more) surfaces of the distal end of a probe 120. By way ofexample, electrode terminals 58 a may be located on a lateral surface,spaced-apart by an electrically insulative matrix 48 a, and electrodes58 b may be located on the distal tip of the probe 120, spaced-apart byan electrically insulating matrix 48 b. The two electrode arrays aresupported by an electrically insulating member 82 preferably havingrounded atraumatic edges 80 to prevent any unwanted mechanical damage(e.g., abrasion) to the tissue being treated. As described in theprevious embodiments, a common electrode structure 17 is disposedproximal to these electrode arrays to provide for the return electricalcurrent path.

Yet another embodiment of the present invention is illustrated in FIGS.10 and 11. In this embodiment, an electrically insulating shield 74 (ormember with electrically insulative coating) extends beyond an array ofelectrode terminals 58 having a width, length and thickness suitable toallow the electrode terminals 58 at the tip of probe 140 to engage abody structure 52 (e.g., meniscus) while preventing any current flow andrelated damage to another closely position body structure 92 (e.g., thenearby articular cartilage 92 located on the surface of the condyle 90).In this manner, the array of electrode terminal 58 can be brought intoclose contact with the target tissue 52 without endangering any criticalbody structures nearby. By way of example, shield 74 may be a metal tabor extension from the crone body which is covered or coated withelectrical insulation. Alternatively, the spatula shaped member 74 maybe formed by injection molding as an integral portion of the insulatinginsert 14.

Yet another embodiment is illustrated in FIG. 12 and is designed forcutting of body structures. In this embodiment, the electrode terminals58 are arranged in a linear or columnar array of one of more closelyspaced columns so that as the electrodes 58 are moved along the longeraxis (denoted by arrow 160 in FIG. 12), the current flux lines arenarrowly confined at the tip of the electrode terminals 58 and result ina cutting effect in the body structure being treated. As before, thecurrent flux lines 60 emanating from the electrode terminals 58 passthrough the electrically conducting liquid to the common electrodestructure 17 located proximal to the probe tip.

Referring now to FIGS. 13, 14 and 15, several alternative geometries areshown for the electrode terminals 58. These alternative electrodegeometries allow the electrical current densities emanating from theelectrode terminals 58 to be concentrated to achieve an increasedablation rate and/or a more concentrated ablation effect due to the factthat sharper edges (i.e. regions of smaller radii of curvature) resultin higher current densities. FIG. 13 illustrates a flattened extensionof a round wire electrode terminal 58 which results in higher currentdensities at the edges 180. Another example is shown in FIG. 14 in whichthe electrode terminal 58 is formed into a cone shaped point 182resulting in higher current densities at the tip of the cone. Yetanother example is shown in FIG. 15 in which the electrode 58 is asquare wire rather than a round wire. The use of a square wire resultsin high current densities along each edge 184 which results from thejuncture of adjacent faces.

Referring to FIG. 16, a high frequency power supply 28 comprises avoltage source 98 which is connected to a multiplicity of currentlimiting elements 96 a, 96 b, . . . ,96 z, typically being inductorshaving an inductance in the range from 100 to 5000 microhenries, withthe particular value depending on the electrode terminal dimensions, thedesired ablation rates, and the like. In the case of ablation ofarticular and fibrocartilage, suitable inductances will usually be inthe range from 50 to 5000 microhenries. Capacitors having capacitancevalues in the range from 200 to 10,000 picofarads may also be used asthe current limiting elements.

Current limiting elements may also be part of a resonant circuitstructure having a capacitor 101 in series with the electrode terminaland an inductor 103 between the electrode lead and the common lead, asillustrated in FIG. 16A. The inductor and capacitor values are selectedaccording to the operating frequency of the voltage source 98. By way ofexample, at an operating frequency of 100 kHz, current limiting circuitstructures may incorporate inductor/capacitor combinations such as (1)2530 microhenries and 1000 picofarads; (2) 5390 microhenries and 470picofarads; or (3) 11,400 microhenries and 220 picofarads, respectively.

It would also be possible to use resistors as the current limitingelements. The use of resistors, however, is generally less preferredthan use of inductors or capacitor/inductor tuned circuit structuressince resistors will have significant IR² power losses which aregenerally avoided with the circuits of FIGS. 16 and 16A.

Referring to FIGS. 1, 16, and 16A, each of the individual leads 97 fromthe current limiting elements 96 and 101/103 are removably connected toleads 92 in cable 24 via connector 26. A common electrode lead 99 fromvoltage source 98 is removably connected to lead 94 in cable 24 via thesame connector 26. Each of the electrode leads 92 and common electrodelead 94 in cable 24 extend into and through handle 22 and terminate inconnector 20 located at the distal end of handle 22. As described withreference to FIGS. 3 and 4, electrical leads 92 and common electrodelead 94 connect to electrode leads 42 and common electrode lead 44,respectively, via the interface between removably attachable connectors19 and 20. In this manner, each of the electrodes in array 12 can bepowered by a single voltage source 98 with independent current limitingelements or circuit structures attached to each electrode lead 42 viacable lead 92 and controller lead 96.

Current limitation could alternatively be accomplished by providing aseparate power supply and current measuring circuitry for each electrodeterminal. Current flow to any electrode terminal which exceeds apreselected (or adjustable) limit would be decreased or interrupted.

Another embodiment of the probe of the present invention intended forcutting or ablation of body structures surrounded by electricallyconducting liquid (i.e., isotonic saline irrigant) is shown in FIG. 17.Two pairs of electrode terminals 58 a/59 a and 58 b/59 b havingflattened tips are shown. Leads 58 a and 58 b are electrically insulatedfrom each other and are individually connected to a separate powersource or common voltage supply with independent current limitingcircuitry, as discussed above. If an independent power source is usedfor each pair, then current flow between the electrodes in pair 58 a and59 a as well as between the electrodes in pair 58 b and 59 b. If theleads 44 a and 44 b from electrodes 59 a and 59 b are connected to acommon electrode 99 in power supply 28, then current will flow betweenelectrode terminals 58 a/58 b and electrodes 59 a/59 b. In particular,current will also flow between electrode 58 b and 59 a. The lineararrangement of electrodes in FIG. 17 is particularly well-suited torapid cutting of body structures while restricting current flux lines 60to the near vicinity of the tip of the probe.

Another embodiment of the probe of the present invention is illustratedin FIG. 18. The probe 200 contains a flexible distal region 112 whichcan be deflected relative to a longitudinal axis 114. Such deflectionmay be selectively induced by mechanical tension, by way of example,through axial translation of thumb slide 108 located on handle 22 which,in turn, increases or decreases tension on a radially offset pull wire110 connected between the slide and the distal end of the probe 200.Alternatively, a shape memory wire may be used which expands orcontracts in response to temperature changes in the wire induced byexternally applied heating currents. Thumb slide 108 could be connectedto a rheostat (not shown) which would control a heating current throughthe wire 110, causing the wire to expand or contract depending on thelevel of the applied current. Said controllable deflection meansimproves access to body structures in certain surgical situations.

Although the foregoing invention has been described in some detail byway of illustration and example, for purposes of clarity ofunderstanding, it will be obvious that certain changes and modificationsmay be practiced within the scope of the appended claims.

What is claimed is:
 1. An electrosurgical instrument for applying electrical energy to a tissue structure at a target site, the probe comprising: a shaft having a proximal end a distal end, where the distal end has a contact surface; an active electrode secured to the shaft and having an exposed portion extending from the distal end of the shaft, such that the exposed portion forms a protrusion on the distal end contact surface; a return electrode coupled to the shaft and having an exposed fluid contact surface where the return electrode is located circumferentially about the shaft and has a substantially greater surface area than the active electrode; at least one connector disposed near the proximal end of the shaft for electrically coupling the active and return electrodes to a high frequency voltage source; and wherein the exposed fluid contact surface of the return electrode is axially spaced from the exposed portion of the active electrode by a distance of at least 1 mm.
 2. The instrument of claim 1 wherein the exposed fluid contact surface of the return electrode is axially spaced from the exposed portion of the active electrode by a distance of about 1 to 2 mm.
 3. The instrument of claim 1 wherein the exposed fluid contact surface of the return electrode is axially spaced from the exposed portion of the active electrode by a distance of about 1.0 to 5.0 mm.
 4. The instrument of claim 1 wherein the return electrode has an exposed surface area at least 5 times greater than an exposed surface area of the active electrode.
 5. The instrument of claim 1 wherein the active electrode comprises a single active electrode disposed near the distal end of the shaft.
 6. The instrument of claim 1 wherein the active electrode includes an array of electrically isolated active electrodes disposed near the distal end of the shaft forming a plurality of protrusions on the distal end contact surface.
 7. The instrument of claim 6 further comprising a plurality of current limiting elements for controlling current flow from at least two of the active electrodes based on electrical impedance between each active electrode and the return electrode.
 8. The instrument of claim 1 wherein the return electrode is spaced from the active electrode by an electrically insulating member comprising an inorganic material.
 9. The method of claim 8 wherein the inorganic material is selected from the group consisting essentially of ceramic, glass and glass/ceramic compositions.
 10. The instrument of claim 1 wherein the active electrode and the return electrode are configured to effect the electrical breakdown of tissue in the immediate vicinity of the active electrode when high frequency voltage is applied between the active electrode and the return electrode in the presence of electrically conducting fluid.
 11. The instrument of claim 1 wherein the exposed fluid contact surface of the return electrode is axially spaced from the exposed portion of the active electrode by a distance of at least 2 mm. 